It is the practice to subject satellites to a test program before they are placed in orbit wherein outer-space conditions among others are simulated. For this purpose, the satellite is placed in a vacuum chamber and is then irradiated intensively on one side. The source of radiation is disposed outside of the vacuum chamber which is provided with a radiation window through which the satellite can be illuminated. In known testing apparatus, the radiation window is a one-piece flat glass plate.
The light power required to simulate the sun which passes through the radiation window is in the order of magnitude of several hundred kilowatts. It is therefore unavoidable that the radiation window becomes heated as a consequence of the unpreventable absorption of a portion of the radiation. The glass from which the window is made must have the smallest possible thickness and a good transparency in order to hold the light loss and the thermal loading of the window as small as possible. However, glasses with the smallest absorption are not available in the large diameter needed for the window. Furthermore, the pressure difference at the window with large diameters requires that also a relatively large minimum thickness be maintained because of reasons of stability.
Accordingly, opposed requirements are placed upon the window with respect to optical and mechanical viewpoints which make it into a critical component. In the event of breakage of the window, for example, because of thermal or mechanical tension in the glass, an explosive pressure equalization through the cross-section having an order of magnitude of one square meter and defined by the window can be expected which can lead to considerable damage.